You have four ways to acquire officers:

1. Recruit Officer
You can get officers through Recruit Officer.
Once in a while, you will get the free chance to recruit officers.
Besides, you can cost Gold and event items to recruit officers.
You may get a whole officer or officer fragments through Recruit Officer.

2. Fuse Fragments

When you collect enough officer fragments, you can fuse them into a whole officer in Officer Gallery. The fused officers are all at initial state.

3. Events
Officers or officer fragments may be rewarded in the events.
Join the events to get them!
 
4. Purchase Packs
Purchasing Packs is a short cut to get powerful officers.